ADAS Boxworth Research Centre, Boxworth, Cambridge CB3 8NN, UK

Background and objectives
Canker (Leptosphaeria maculans) remains a major cause of yield loss in winter oilseed rape in the UK, in spite of an increase in fungicide use [1]. In eastern England, many crops have 80-100% plants with moderate or severe cankers equating to a yield loss of 1 t/ha. Good control of canker had been achieved with intensive programmes of a mixture of the fungicides prochloraz+iprodione+thiophanate-methyl [2], but little information is available to define minimal dose requirements. The results of experiments designed to determine the effectiveness of programmes at full and half dose in relation to epidemic development are presented in this paper.

Materials and methods
Experiments were carded out in commercial crops of winter oilseed rape, at ADAS Boxworth, Cambridge in 1994-95 and 1995-96, and at Ripon, North Yorkshire in 1994-95, on the canker-susceptible cv. Rocket. A high-disease site with continuous cropping of winter oilseed rape since 1988 was used at ADAS Boxworth. The fungicides difenoconazole, flusilazole+carbendazim, prochloraz+carbendazim and tebuconazole were applied as either four-spray programmes (4-6 week interval between treatments) over the period late October-early March, or two-spray programmes (late October/early November+early March) at both full label dose rate or half label dose in a randomized block design with three replicates. Disease incidence and severity (percentage leaf area affected or stem disease index; 0=healthy, 4=dead) were assessed at each spray date on 10 plants per plot, 4 weeks after the last spray and pre-harvest. Plot yields were taken with a plot combine and adjusted to 90% dry matter.

Results and conclusions
At Boxworth, phoma leaf spot increased from 17% plants affected on 17 October 1994 to 100% plants affected on 11 November when the first sprays were applied. Treatments protected plants from further leaf spotting for 6-8 weeks, but only difenoconazole and fiuzilazole+carbendazim applied four times at either full or half dose provided good (>60%) control of canker, which was severe at harvest (untreated index 3.1). At Ripon, fungicides were applied as leaf spotting appeared (20% plants affected, 17 November), and under moderate disease pressure (55% plants with canker on 20 June) all fungicides gave good control of canker with four sprays at full or half dose, and all except tebuconazole were effective when two sprays were applied at full dose. All fungicides gave <35% control when applied twice at half dose. Maximum yield increases were 0.8 t/ha at both sites (24% response at Boxworth and 16% at Ripon). In 1995-96, first sprays at Boxworth were applied 11 days after 100% of plants had phoma leaf spot and canker control was poor, the most effective treatment (flusilazole+carbendazim at four full doses) gave only 45% control of canker. The large yield increases (up to 1.6 t/ha above 1.6 t/ha control) obtained at this site were associated with increased plant survival during the winter.

These experiments demonstrated that fungicides had limited curative activity once phoma leaf spot appeared in the crop. The timing of the first sprays was critical and fungicides were most effective when used as protectants with a total of at least two full doses. It appeared that difenozonazole and flusilazole+carbendazim had greater curative activity than prochloraz+carbendazim and tebuconazole. These results are consistent with fungicides controlling the pathogen within the leaf or as it grew within the petiole, but not after it had reached the stem base. The failure to apply fungicides in the autumn at the onset of the phoma leaf spot epidemic appears to be the main reason for poor control on farms, despite repeated applications of fungicides [1].

1. Fitt BDL, Gladders P, Turner JA et al., 1997. Aspects of Applied Biology 48, 135-142.
2. Sansford CE, Fitt BDL, Gladders P et al., 1996. Home-Grown Cereals Authority Project Report OS17.